It is sometimes requested to measure the number of bacteria included in food for the purposes of food hygiene management and the like. A conventional method of measuring bacteria included in a specimen such as food dilutes the specimen in stages and applies them in a fixed amount on an agar medium, cultures them for about 24 hours, and visually calculates the number of derived colonies, to measure the number of bacteria. Unfortunately, this method has the need for staged specimen dilution and the need for about 24-hour culture. This resulted in the development of a method that measures the number of bacteria by measuring dissolved oxygen concentration contained in a liquid medium added with a specimen with an oxygen electrode (hereafter also called an oxygen electrode method), as is described in Japanese Patent Application Laid-Open No. 2000-287699.
In the oxygen electrode method described in Japanese Patent Application Laid-Open No. 2000-287699, the higher the dissolved oxygen concentration contained in the liquid medium, the larger amount of current is measured. Bacteria included in the specimen consume the dissolved oxygen in the liquid medium through respiration. As the dissolved oxygen concentration decreases due to the respiration of the bacteria, a current flowing through the oxygen electrode decreases. In addition, the amount of dissolved oxygen consumed depends on the initial number of bacteria included in the specimen. Namely, the larger the initial number of bacteria, the larger amount of oxygen is consumed and the faster the dissolved oxygen concentration decreases. When the dissolved oxygen concentration decreases in a short time, the current value being measured also decreases in a short time. That is, the time required for a current flowing through a liquid medium including a specimen with an unknown initial number of bacteria to decrease down to a predetermined threshold value is obtained, thereby determining the initial number of bacteria corresponding to the time required. In such ways, the oxygen electrode method is able to measure the initial number of bacteria in a short time and accurately.
The oxygen electrode method such as is described in Japanese Patent Application Laid-Open No. 2000-287699 encounters a situation where a current value being measured does not decrease down to a threshold value or below depending on the combination of a medium and a bacterial strain. Such situation causes the occurrence of false detection such as a false negative and variations in detection times, resulting in a reduction in measurement accuracy.
Another problem of the conventional oxygen electrode methods is that fungi cannot be measured accurately because fungi have a slow respiration rate and thus when cultured in a typical manner, the oxygen concentration does not decrease rapidly. Further, fungi have a slow growth rate and can be determined by the conventional oxygen electrode methods only after growing for about a week, resulting in a prolonged measurement time.